The present invention relates to magnetic storage devices and, more particularly, to a method for overcoming a stiction condition in a disk drive.
FIG. 1 illustrates a perspective view of a conventional disk drive 10. The disk drive 10 includes at least one disk 12 that is rotated by a spin motor 14. The spin motor 14 is mounted to a base plate 16. An actuator arm assembly 18 is also mounted to a base plate 16.
The actuator arm assembly 18 includes a head 20 mounted to a flexure arm 22 which is attached to an actuator arm 24 that can rotate about a bearing assembly 26. The actuator arm assembly 18 also contains a voice coil motor 28 which moves the head 20 relative to the disk 12. The spin motor 14, voice coil motor 28 and head 20 are coupled to a number of electronic circuits 30 mounted to a printed circuit board 32. The electronic circuits 30 typically include a read channel chip, a microprocessor-based controller and a memory device (e.g., a random access memory (RAM) device and a read only memory (ROM) device). It should be understood that typical disk drives (like disk drive 10) normally include a plurality of disks 12 and, therefore, a plurality of corresponding actuator arm assemblies 18.
FIG. 2 is a simplified diagrammatic representation of perspective view of a head 20 showing its air-bearing surface, wherein the air-bearing surface is comprised of three pads 21a, 21b and 21c. As shown in FIG. 2, the head 20 includes an air-bearing slider 34 and a transducer 36.
When the spin motor 14 rotates the disk 12, the head 20 floats above (or below) the disk 12 on a small cushion of air due to the aerodynamic characteristics of the air-bearing slider 34. When the disk 12 is brought to a stopped condition, there is no aerodynamic cushion available to float the head 20 above the disk 12 and, therefore, the head 20 lands on the disk 12. Consequently, to prevent damage to disk 12, each disk surface 38 (see FIG. 3) includes a landing zone 40 onto which the head 20 is usually parked. The landing zone 40 is usually a textured area of the disk surface 38 where data is not normally stored. In contrast, the disk surface 38 also includes a data zone 42, which is designed to be extremely smooth for a number of reasons which are not of particular significance for this application. Once the head 20 has been parked in landing zone 40, a latch (not shown) is used to lock the head 20 over the landing zone 40 to prevent undesired movement of the head 20 from the landing zone 40 onto the data zone 42 due to shocks to the drive (e.g., dropping or bumping the drive). In some instances, shocks experienced by the disk drive 10 are such that the latch fails. In these instances, the head 20 may still be in the landing zone 40 without the latch in place or the head 20 may be displaced into the data zone 42.
Typically, once a head 20 has been parked in the landing zone 40, the spin motor 14 is activated which has a torque which is generally sufficient to lift the head 20 into a flying condition over the disk surface 38. In certain instances, however, the head 20 adheres to the disk surface 38 in such a manner that the torque of the spin motor 14 is insufficient to lift the head 20 into a flying condition. This phenomenon is known as stiction.
Stiction may occur when the head 20 is either at rest in the landing zone 40 (whether latched or unlatched) or when the head 20 is at rest in the data zone 42. Generally, stiction is a greater problem when the head 20 is at rest in the data zone 42 since the data zone 42 is smooth and, therefore, the contact area between the head 20 and the data zone 42 is large (as compared to the textured landing zone 40). It must be noted, however, that stiction may also occur in the landing zone 40, for example, when lubricants normally found on the disk surface 38 have migrated to the interface between the head 20 and the disk surface 38 causing a meniscus effect. Other reasons for stiction are well-known in the art.
Because stiction is a significant problem and may render a disk drive inoperable, a number of techniques have been devised in an attempt to overcome stiction conditions. For example, U.S. Pat. No. 4,530,021 entitled xe2x80x9cMicromotion Release of Heads from Lubricated Magnetic Disksxe2x80x9d discloses a generally triangular waveform of relatively short duration that is supplied to the coils of the voice-coil motor prior to supplying power to rotate the disks. According to FIG. 1 of U.S. Pat. No. 4,530,021, the generally triangular waveform may also be applied while power is supplied to rotate the disks.
Another example is found in U.S. Pat. No. 5,397,971 entitled xe2x80x9cBi-Polar Disk Torquing System for a Disk Drive to Free Stuck Transducers.xe2x80x9d U.S. Pat. No. 5,397,971 discloses a method of exciting the spin motor with bi-polar pulses of direct current at substantially the resonant frequency of the rotating assembly to free stuck transducers. The back EMF is sensed from the spin motor to detect rotation.
Another example is found in U.S. Pat. No. 5,384,675 entitled xe2x80x9cDisk Drive With Controlled Actuator Oscillation for Release of Head Carriers.xe2x80x9d U.S. Pat. No. 5,384,675 discloses applying a series of alternating current pulses to an actuator to dither head carriers at an initial frequency that is generally equal to or above the resonant frequency of the actuator systems with stuck carriers. The frequency and amplitude of the pulses are swept downward from the initial frequency and amplitude, and is repeated until the drive motor rotates at its operating speed.
Yet another example is found in U.S. Pat. No. 5,530,602 entitled xe2x80x9cDisk Drive Micromotion Starting Apparatus and Method.xe2x80x9d U.S. Pat. No. 5,530,602 discloses a method of applying a series of alternating current bursts to the voice coil motor, beginning with a lowest amplitude burst. Each succeeding current burst is of increased amplitude. During each burst, the pulse frequency is varied progressively through a range of values, wherein the range of selected frequencies are selected to enable the frequency of some of the burst pulses to approach the resonant frequency of one or more modes of disk vibration. A sensor connected to the spindle motor coils responds to the presence of a back EMF, indicating that the heads are no longer adhered to the disk surfaces and that rotation has begun.
Yet a further example is found in U.S. Pat. No. 5,801,505 entitled xe2x80x9cDisk Drive Stiction Release System.xe2x80x9d U.S. Pat. No. 5,801,505 discloses a method of pulsing the spindle motor and/or the actuator motor if stiction occurs during start-up. Because the resonant frequency of the spindle motor varies depending upon the number of heads that are stuck, pulses are applied over a varying range of frequencies, including the various resonant frequencies of the spindle motor corresponding to various numbers of heads stuck.
It should be understood that the above discussion of the aforementioned patents is only intended to be a brief discussion of each of the patents. To obtain a full understanding of the entire disclosure of such patents and the information contained therein, reference should be made to the patents themselves.
While the above patents provide a number of different alternatives for overcoming stiction conditions, it would be beneficial to provide a relatively simple yet effective technique for overcoming stiction conditions. In addition, it would be beneficial to provide a technique which reduces occurrences of re-stiction.
The present invention is designed to overcome the aforementioned problems and meet the aforementioned, and other, needs.
A method for overcoming a stiction condition in a disk drive is disclosed. In one embodiment, once a stiction condition is sensed due to non-movement of the spin motor, a microprocessor controls the application of a first oscillation current to a voice coil motor in order to dither stuck heads. The first oscillation current has a fixed frequency which corresponds to the resonant frequency of the lateral bending modes of the disk drive. The first oscillation current has an amplitude which, in combination with forces generated by the spin motor, has a force that roughly corresponds with freeing one stuck head. If application of the first oscillation current causes the spin motor to begin rotation, the first oscillating current continues to be applied for at least one commutation state of the spin motor, so as to minimize occurrences of re-stiction.
If the spin motor has failed to begin rotation after application of the first oscillating current, a second oscillating current is applied having a frequency equal to the fixed frequency and a second amplitude greater than the first amplitude. The second oscillating current may have a second amplitude which, in combination with forces generated by the spin motor, has a force that roughly corresponds with freeing three stuck heads. If application of the second oscillation current causes the spin motor to begin rotation, the second oscillating current continues to be applied for at least one commutation state of the spin motor, so as to minimize occurrences of re-stiction.
If the spin motor has failed to begin rotation after application of the second oscillating current, the process may be repeated. In each case, however, the amplitude of the nth oscillating current is greater than amplitude of the (nxe2x88x921)th oscillating current, but the frequency of each of the oscillating currents is equal to the fixed frequency.
In one embodiment, one or more of the oscillating currents are sinusoidal waveforms, which excite fewer resonances and, therefore, are less audible. To reduce the burden on the microprocessor and to avoid calculation delays, the values of the sinusoidal waveform are preferably stored in a look-up table.